The aim of these studies is to characterize in detail several of the specific mechanisms that operate to control the rate at which cholesterol is synthesized by the liver. These studies will be directed at three different aspects of the feedback, regulatory mechanisms including identification of effector(s) of regulation, characterization of the membrane transport processes responsible for movement of the effector(s) into the cell and characterization of the mechanisms responsible for alteration in the activity of HMG CoA reductase. Using intact animals, the possible role of various hormones and of steroids such as cholesterol, cholesterol esters and other cholesterol metabolites as effector(s) of the rate of hepatic cholesterogenesis will be evaluated. Studies also will be undertaken to characterize the membrane translocation step whereby lipids move from various lipoprotein fractions into the hepatic cell. These will include studies to define partitioning coefficients of lipids between aqueous phase and lipoprotein and the mechanism(s) of membrane translocation of various lipids. Other studies will define the approximate thickness and functional area of unstirred layers outside the cell membranes and identify the relative resistance engendered by these layers during the movement of molecules into the liver cell. Analysis also will be made of the manner in which the carrier, i.e., lipoproteins, interacts with the hepatocyte membrane. Finally, we will also undertake further purification of solubilized HMG CoA reductase and attempt to develop a specific antibody for this enzyme. Use of such an antibody combined with appropriate kinetic studies should provide additional information on which of the diverse physiological maneuvers known to affect cholesterol synthesis such as cholesterol feeding, stress, diurnal lighting, etc., alter synthesis of cholesterol by altering the rate of enzyme synthesis, by changing the rate of enzyme degradation or by direct feedback inhibition of HMG CoA reductase.